For the marinade, in a small mixing bowl, combine 1/4 cup of the oil, the lemon and tangerine juices, the wine, mustard, Grand Marnier, brown sugar, paprika, red pepper flakes, onion, 2 of the thyme sprigs, 1 1/2 teaspoons of salt, and 1/2 teaspoon of pepper.Place the chicken in a large plastic bag. Pour in the marinade, seal, and turn to coat completely. Marinate in the refrigerator for at least 1 hour or overnight.Remove the chicken from the refrigerator, ideally 1 hour before cooking if you have marinated it overnight. Set racks in the middle and upper third of the oven. Preheat the oven to 450ºF. Place a colander over a large mixing bowl and drain the chicken, reserving the marinade along with the onion and thyme. Remove the chicken and dry thoroughly with paper towels. Pour the marinade, onion, and thyme into a rimmed baking sheet.In a large skillet, heat 1 tablespoon of the remaining oil over medium-high heat until shimmering. Add half the chicken pieces skin side down (do not crowd them) and reduce the heat to medium. Cook for 4 to 5 minutes, undisturbed, until a dark golden crust forms. Remove the chicken from the skillet and place on the baking sheet, skin side up, on top of the marinade.Wipe the skillet clean. Add the remaining tablespoon of oil and repeat with the remaining chicken. Transfer the chicken to the baking sheet, reserving the oil in the skillet. Turn off the heat and let the skillet cool for 1 minute.

Add the garlic to the oil in the skillet and turn the heat to medium. Cook the garlic for 3 minutes, flipping it halfway through until the garlic is lightly browned on both sides. Transfer the garlic to the baking sheet with the chicken.

Arrange the lemon and tangerine slices around and under the chicken. Lay 3 sprigs of thyme on top and season with salt and pepper. Bake on the middle rack for 25 to 30 minutes, until the internal temperature reaches 165°F on an
instant-read thermometer and the juices run clear.

Remove the baking sheet from the oven and raise the temperature to broil. Transfer the chicken pieces to a serving platter, leaving the marinade, citrus, onion, and garlic on the sheet. Broil on the upper rack for 4 to 5 minutes, until
the citrus slices caramelize. Remove the sheet from the oven and arrange the citrus, garlic, and onion under, on, and around the chicken. Garnish with a few sprigs of thyme.

Pour the liquid from the baking sheet into a small saucepan over medium-high heat and bring to a boil. Reduce the heat to low and simmer until the sauce reduces by a third, 8 to 10 minutes.